If you’re using email marketing you need to aware of this new Gmail feature where it notifies email receivers if your email was encrypted. If it isn’t encrypted a red open padlock icon appears:
And on mouseover the user is told this message isn’t encrypted:
Most ISPs should be taking care of this but if your mailing list is seeing the open padlock look into fixing this. More details– https://support.google.com/mail/answer/6330403?p=tls&hl=en&rd=1